In the oil and gas industry, risers are a well known and widely used piece of equipment for offshore drilling and production. Risers are generally described as individual conduits that can be axially interconnected to provide a continuous conduit between a subsea oil well to a surface drilling/production facility.
Risers are used in both the drilling phases of an offshore drilling well as well as the production phase. During drilling, interconnected risers are used to surround a drilling string as the drilling string moves up and down and rotates during the downholedrilling and upholetripping phases. As such, the riser provides a means of encasing and protecting the drill string as well as to also contain circulating drilling fluid across the distance between the seafloor and surface drilling rig. During production, a riser will primarily serve as the conduit between the well and the on-surface production vessel. Risers may also provide a primary structural interconnection between the drilling platform on the sea surface and the ocean floor.
As such, risers are engineered to withstand the various loads required by the foregoing functions. That is, a riser must withstand substantial internal pressures from the internal pressure of drilling or production fluids being pumped within the drill string and within the riser. Risers must also withstand axial tension loads from the interconnected weight of a string of assembled risers that are being moved into and out of position prior to and after drilling. In addition, risers must also withstand axial compressive and bending loads that are imparted to a string of risers during use as a drilling rig moves with respect to the ocean floor. FIG. 1 shows the general construction of an offshore drilling rig (described in greater below).
Increasingly, offshore drilling rigs are also being operated in deeper water than in the past, and as a result, the length of riser strings are similarly increasing. Accordingly, the above described loading forces on the risers are also increasing with the need for risers to withstand greater loads.
As offshore drilling rigs were initially developed, first generation risers were manufactured as unitary steel structures and simply comprised a hollow steel tube having an outer wall and with appropriate threaded or flanged connectors at either end to enable interconnection of individual risers. The main problems with steel risers included the relative weight of individual risers for the required loading such that as the loading on the risers increased, they became progressively more unwieldy in terms of weight. That is, with increasing weight, there were associated costs and complexities of handling large and heavy tubes particularly in an offshore environment. As a result, later generation composite risers have been developed and have been in use for many years.
In the typical composite riser design, the riser includes a relatively thin internal steel liner surrounded by an outer composite jacket. At either end of the riser, industry standard steel fittings are continuous with and/or are connected to the thin steel liner to enable adjacent risers to be connected to one another. As such, the internal steel portion of the riser includes a relatively thin central portion that either tapers towards or is connected to a thicker end portion that comprises the connectors. Similarly, the composite outer jacket has a thicker central portion that surrounds the thin central steel portion that tapers to a thinner end portion that is engaged with the connector portions.
The junction between the steel and composite material is engineered to ensure that the structural loads between the steel and composite materials are effectively transmitted between the two materials over time. More specifically, as a result of differences in the stiffness of steel and composite structures, the forces are transmitted over a smaller area as compared to materials that may have similar stiffnesses. This mismatch in stiffnesses can be addressed by effectively increasing the areas of contact between the two materials. As shown in FIG. 2 and as described in U.S. Pat. No. 6,719,058 (incorporated herein by reference) a typical metal/composite interface (MCI) is shown and is often referred to as “traplock” design. In this design, the tapering metal end is provided with one or more integral rings or grooves transverse to the longitudinal liner axis (i.e. in a sagittal plane). When the internal steel components are wrapped with composite material, which may include a combination of axial, transverse and angled resin-impregnated fibers, the axial forces (compressive and tensile) are more effectively transferred across the interface.
More specifically, axially oriented reinforcing fiber is held in the trap by circumferentially wound fiber. The tapered interface of the metal/composite fitting transfers load through bearing pressure. The load is distributed from the metal/composite interface to the axial fibers by shear. Generally, a high hoop stiffness and strength is needed in the area behind the metal/composite interface to prevent the axial fibers from being pulled out of the laminate or the composite from expanding and sliding over the top of the tapered interface.
While the foregoing has been an effective design, there remain limits regarding the ultimate strength of such systems. For example, as shown in FIG. 2, finite element analysis of a typical junction reveals that there remain significant stress points at particular positions on the grooves as the axial compressive or tensile loads increase.
As a result, there has been a need for improved metal/composite interfaces particularly for risers.